Having fought her way to the top of British politics, new Prime Minister Theresa May will face an even tougher battle in power: plotting a divorce from the European Union that she once opposed but now says must not be halted. The fate of the post-World War Two project of European integration, and even of the United Kingdom itself, will depend on May's rapport with another pastor's daughter who climbed to the summit of a conservative party: Germany's Angela Merkel. May's biggest immediate decision will be when to trigger formal exit proceedings. It is a call she will have to make while also facing an economy under extreme strain, deep division in the ruling party and potential demands from nationalists in Scotland for a referendum on independence. Despite having campaigned to remain in the EU before the June 23 Brexit referendum, May, 59, said she would steer Britain through a period of great national change as it left the EU and build an economy that worked not only for a privileged few. "We will forge a bold new positive role for ourselves in the world, and we will make Britain a country that works . for every one of us," the former interior minister said outside 10 Downing Street after Queen Elizabeth asked her to form a government. The Brexit vote thrust Britain into political crisis with both major parties in leadership battles and investors left guessing about the future relationship with the EU. Despite calls from Brexiteers and many European leaders to start the divorce process as soon as possible, May has said she will not trigger Article 50 of the Lisbon Treaty, which starts a two-year countdown to exit, before the end of this year. As she entered Downing Street in place of her former boss David Cameron, who announced his resignation after the Brexit vote, opponents of the EU protesting outside chanted "What do we want? Brexit. When do we want it? Now". "IRON MAYDEN" As Britain's second female prime minister, May has prompted comparisons with "Iron Lady" Margaret Thatcher, who governed from 1979 until 1990 and was no stranger to doing battle in Europe. May's six years in charge of the cabinet's law-and-order portfolio was the longest tenure for a century in what is widely seen as one of the government's toughest jobs. The Sun newspaper, Britain's biggest selling daily, cast May as "The Iron Mayden", the new "Mrs T". Veteran Conservative lawmaker Ken Clarke, who served as a minister under Thatcher, was caught on a live microphone in a TV studio this month describing May as "a bloody difficult woman". May took the remark as a badge of honour, repeating it to lawmakers at a party leadership hustings, and adding that the next person who would find out just how difficult she was would be European Commission head Jean-Claude Juncker. The crucial issue is balancing the desire of British voters to restrict immigration from other EU countries with the need of British business to keep access to the European common market. May has a record of negotiating compromises with the EU, but will be up against Merkel, who has been chancellor for 10 years, often outlasts other EU leaders at late night meetings and, like them, has said there can be no "cherry picking" by Britain: it must accept free movement if it wants free trade. She appointed David Davis, who campaigned for Britain to leave the EU, as Secretary of State for exiting the bloc and former London Mayor Boris Johnson, a late joiner and subsequent leader of the "Leave" campaign, as foreign minister. "VERY PERSISTENT" The Oxford University-educated daughter of a Church of England vicar, May is highly regarded among European officials, who say she knows Brussels well and is always well prepared. "She won't be an easy partner for the EU," said one senior EU official familiar with negotiations in which May has taken part, adding that she does not change her tune easily. "She's been extremely consistent, very persistent." Colleagues say May, who became a lawmaker in 1997, prefers to spend her free time with her husband of 36 years, Philip, rather than among the "old boys club" in parliament's bars. That is a relief for her party and the wider country after a Brexit debate and bitter post-referendum leadership contest dominated by infighting among male Conservatives who had been friends and rivals since boarding school and university. "Frankly at the moment most people in this country would be quite pleased at the thought they've got a prime minister who doesn't gossip, doesn't do back-stabbing," Anne Jenkin, a Conservative member of the upper house who with May co-founded a group to try to get more women in to parliament, told the BBC. Former Belgian interior minister Joelle Milquet said that while she did not share May's views on Europe, she found her to be "pleasant and warm" on a personal level, and someone who defends her position "with panache". "She is a very level-headed woman, who is a good judge of things," she told Reuters. SERIOUS CHANGE Despite similarities to fellow pragmatist Merkel, May has shunned comparisons with other female leaders. "My pitch is simple - I'm Theresa May and I think I'm the best person to lead this country." In her first public comments as prime minister, May said she understood the "burning injustice" that existed in Britain and pledged to give hardworking people more control. She also said she believed in the "precious bond between England, Scotland, Wales and Northern Ireland". During the leadership contest, she had demanded the government scrap its plan for a budget surplus by 2020, easing the fiscal plans that were the bedrock of David Cameron's six years in power. "The only surprise is that there is so much surprise in Westminster about the public's appetite for change," May told supporters and journalists in Birmingham on Monday. "And make no mistake, the referendum was a vote to leave the European Union, but it was also a vote for serious change." The first cabinet announcement by her office was the replacement of Finance Minister George Osborne, synonymous in many voters' minds with austerity, with foreign minister Philip Hammond. "STAKHANOVITE" As interior minister, May opted back into a European arrest warrant system and cross-border information sharing despite Britain's 'opt out' on EU justice and home affairs policy. In brokering those 'opt-ins', the senior EU official said: "She took a great part in the negotiations herself, she didn't rely on officials . She has very obvious negotiating skills." Timothy Kirkhope, a Conservative member of the European Parliament and the party's spokesman for home affairs there, said May had picked out the best bits for Britain. "It was very matter of fact, businesslike," said Kirkhope, also a former junior minister in Britain's interior ministry. "She has this ability to not let on too much where her negotiations might be going . If she were a poker player I would probably have some reservations about taking her on." May, who lost both her parents while in her twenties, describes herself as a practising Christian and a fan of cooking, owning more than 100 cookery books. She gets up early and spends time in the gym, said one Conservative lawmaker who has worked closely with her. She has Type One diabetes and needs insulin injections several times a day, once describing in an interview how she been forced to break strict parliamentary rules on not eating in the chamber during a particularly long debate. "I had a bag of nuts in my handbag and one of my colleagues would lean forward every now and then, so that I could eat some nuts without being seen by the Speaker," she said. Despite criticism for a poster campaign telling illegal immigrants to go home, May has largely won plaudits from both colleagues and political opponents during her six years as interior minister. She has pushed through measures including reforms of the police and moves to tackle modern slavery. Damian Green, a Conservative lawmaker who worked under her as a junior minister, described her as "completely Stakhanovite" - a Russian term for someone with legendary capacity for hard work. "She had a very clear sense of long-term direction as well as the capacity to do the detail," Green told Reuters. "She wasn't to be pushed aside or pushed about."